8 files changed, 360 insertions, 130 deletions

diff --git a/embassy-stm32/src/lib.rs b/embassy-stm32/src/lib.rs
index 8c87ea7d5..ec8648ee4 100644
--- a/embassy-stm32/src/lib.rs
+++ b/embassy-stm32/src/lib.rs
@@ -197,6 +197,11 @@ pub fn init(config: Config) -> Peripherals {
         // must be after rcc init
         #[cfg(feature = "_time-driver")]
         time_driver::init();
+
+        #[cfg(feature = "low-power")]
+        while !crate::rcc::low_power_ready() {
+            crate::rcc::clock_refcount_sub();
+        }
     }
 
     p
diff --git a/embassy-stm32/src/low_power.rs b/embassy-stm32/src/low_power.rs
index 7814fa384..65b93f8a4 100644
--- a/embassy-stm32/src/low_power.rs
+++ b/embassy-stm32/src/low_power.rs
@@ -3,46 +3,52 @@ use core::marker::PhantomData;
 
 use cortex_m::peripheral::SCB;
 use embassy_executor::*;
-use embassy_time::Duration;
 
 use crate::interrupt;
 use crate::interrupt::typelevel::Interrupt;
 use crate::pac::EXTI;
 use crate::rcc::low_power_ready;
+use crate::time_driver::{get_driver, RtcDriver};
 
 const THREAD_PENDER: usize = usize::MAX;
-const THRESHOLD: Duration = Duration::from_millis(500);
 
-use crate::rtc::{Rtc, RtcInstant};
+use crate::rtc::Rtc;
 
-static mut RTC: Option<&'static Rtc> = None;
+static mut EXECUTOR: Option<Executor> = None;
 
 foreach_interrupt! {
     (RTC, rtc, $block:ident, WKUP, $irq:ident) => {
         #[interrupt]
         unsafe fn $irq() {
-            Executor::on_wakeup_irq();
+            unsafe { EXECUTOR.as_mut().unwrap() }.on_wakeup_irq();
         }
     };
 }
 
-pub fn stop_with_rtc(rtc: &'static Rtc) {
-    crate::interrupt::typelevel::RTC_WKUP::unpend();
-    unsafe { crate::interrupt::typelevel::RTC_WKUP::enable() };
-
-    EXTI.rtsr(0).modify(|w| w.set_line(22, true));
-    EXTI.imr(0).modify(|w| w.set_line(22, true));
-
-    unsafe { RTC = Some(rtc) };
-}
+// pub fn timer_driver_pause_time() {
+//     pause_time();
+// }
 
-pub fn start_wakeup_alarm(requested_duration: embassy_time::Duration) -> RtcInstant {
-    unsafe { RTC }.unwrap().start_wakeup_alarm(requested_duration)
+pub fn stop_with_rtc(rtc: &'static Rtc) {
+    unsafe { EXECUTOR.as_mut().unwrap() }.stop_with_rtc(rtc)
 }
 
-pub fn stop_wakeup_alarm() -> RtcInstant {
-    unsafe { RTC }.unwrap().stop_wakeup_alarm()
-}
+// pub fn start_wakeup_alarm(requested_duration: embassy_time::Duration) {
+//     let rtc_instant = unsafe { EXECUTOR.as_mut().unwrap() }
+//         .rtc
+//         .unwrap()
+//         .start_wakeup_alarm(requested_duration);
+//
+//     unsafe { EXECUTOR.as_mut().unwrap() }.last_stop = Some(rtc_instant);
+// }
+//
+// pub fn set_sleepdeep() {
+//     unsafe { EXECUTOR.as_mut().unwrap() }.scb.set_sleepdeep();
+// }
+//
+// pub fn stop_wakeup_alarm() -> RtcInstant {
+//     unsafe { EXECUTOR.as_mut().unwrap() }.rtc.unwrap().stop_wakeup_alarm()
+// }
 
 /// Thread mode executor, using WFE/SEV.
 ///
@@ -57,54 +63,62 @@ pub fn stop_wakeup_alarm() -> RtcInstant {
 pub struct Executor {
     inner: raw::Executor,
     not_send: PhantomData<*mut ()>,
+    scb: SCB,
+    time_driver: &'static RtcDriver,
 }
 
 impl Executor {
     /// Create a new Executor.
-    pub fn new() -> Self {
-        Self {
-            inner: raw::Executor::new(THREAD_PENDER as *mut ()),
-            not_send: PhantomData,
+    pub fn take() -> &'static mut Self {
+        unsafe {
+            assert!(EXECUTOR.is_none());
+
+            EXECUTOR = Some(Self {
+                inner: raw::Executor::new(THREAD_PENDER as *mut ()),
+                not_send: PhantomData,
+                scb: cortex_m::Peripherals::steal().SCB,
+                time_driver: get_driver(),
+            });
+
+            EXECUTOR.as_mut().unwrap()
         }
     }
 
-    unsafe fn on_wakeup_irq() {
-        info!("on wakeup irq");
+    unsafe fn on_wakeup_irq(&mut self) {
+        trace!("low power: on wakeup irq");
 
-        cortex_m::asm::bkpt();
+        self.time_driver.resume_time();
+        trace!("low power: resume time");
     }
 
-    fn time_until_next_alarm(&self) -> Duration {
-        Duration::from_secs(3)
-    }
+    pub(self) fn stop_with_rtc(&mut self, rtc: &'static Rtc) {
+        trace!("low power: stop with rtc configured");
 
-    fn get_scb() -> SCB {
-        unsafe { cortex_m::Peripherals::steal() }.SCB
+        self.time_driver.set_rtc(rtc);
+
+        crate::interrupt::typelevel::RTC_WKUP::unpend();
+        unsafe { crate::interrupt::typelevel::RTC_WKUP::enable() };
+
+        EXTI.rtsr(0).modify(|w| w.set_line(22, true));
+        EXTI.imr(0).modify(|w| w.set_line(22, true));
     }
 
-    fn configure_pwr(&self) {
-        trace!("configure_pwr");
+    fn configure_pwr(&mut self) {
+        trace!("low power: configure_pwr");
 
+        self.scb.clear_sleepdeep();
         if !low_power_ready() {
+            trace!("low power: configure_pwr: low power not ready");
             return;
         }
 
-        let time_until_next_alarm = self.time_until_next_alarm();
-        if time_until_next_alarm < THRESHOLD {
+        if self.time_driver.pause_time().is_err() {
+            trace!("low power: configure_pwr: time driver failed to pause");
             return;
         }
 
-        trace!("low power stop required");
-
-        critical_section::with(|_| {
-            trace!("executor: set wakeup alarm...");
-
-            start_wakeup_alarm(time_until_next_alarm);
-
-            trace!("low power wait for rtc ready...");
-
-            Self::get_scb().set_sleepdeep();
-        });
+        trace!("low power: enter stop...");
+        self.scb.set_sleepdeep();
     }
 
     /// Run the executor.
@@ -126,11 +140,11 @@ impl Executor {
     ///
     /// This function never returns.
     pub fn run(&'static mut self, init: impl FnOnce(Spawner)) -> ! {
-        init(self.inner.spawner());
+        init(unsafe { EXECUTOR.as_mut().unwrap() }.inner.spawner());
 
         loop {
             unsafe {
-                self.inner.poll();
+                EXECUTOR.as_mut().unwrap().inner.poll();
                 self.configure_pwr();
                 asm!("wfe");
             };
diff --git a/embassy-stm32/src/rcc/mod.rs b/embassy-stm32/src/rcc/mod.rs
index 3c75923e5..45a4d880d 100644
--- a/embassy-stm32/src/rcc/mod.rs
+++ b/embassy-stm32/src/rcc/mod.rs
@@ -86,6 +86,8 @@ static CLOCK_REFCOUNT: AtomicU32 = AtomicU32::new(0);
 
 #[cfg(feature = "low-power")]
 pub fn low_power_ready() -> bool {
+    trace!("clock refcount: {}", CLOCK_REFCOUNT.load(Ordering::SeqCst));
+
     CLOCK_REFCOUNT.load(Ordering::SeqCst) == 0
 }
 
diff --git a/embassy-stm32/src/rtc/mod.rs b/embassy-stm32/src/rtc/mod.rs
index a6102077a..1f1abb78d 100644
--- a/embassy-stm32/src/rtc/mod.rs
+++ b/embassy-stm32/src/rtc/mod.rs
@@ -1,6 +1,14 @@
 //! RTC peripheral abstraction
 mod datetime;
 
+#[cfg(feature = "low-power")]
+use core::cell::Cell;
+
+#[cfg(feature = "low-power")]
+use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;
+#[cfg(feature = "low-power")]
+use embassy_sync::blocking_mutex::Mutex;
+
 pub use self::datetime::{DateTime, DayOfWeek, Error as DateTimeError};
 
 /// refer to AN4759 to compare features of RTC2 and RTC3
@@ -30,9 +38,73 @@ pub enum RtcError {
     NotRunning,
 }
 
+#[cfg(feature = "low-power")]
+/// Represents an instant in time that can be substracted to compute a duration
+struct RtcInstant {
+    second: u8,
+    subsecond: u16,
+}
+
+#[cfg(feature = "low-power")]
+impl RtcInstant {
+    pub fn now() -> Self {
+        let tr = RTC::regs().tr().read();
+        let tr2 = RTC::regs().tr().read();
+        let ssr = RTC::regs().ssr().read().ss();
+        let ssr2 = RTC::regs().ssr().read().ss();
+
+        let st = bcd2_to_byte((tr.st(), tr.su()));
+        let st2 = bcd2_to_byte((tr2.st(), tr2.su()));
+
+        assert!(st == st2);
+        assert!(ssr == ssr2);
+
+        let _ = RTC::regs().dr().read();
+
+        let subsecond = ssr;
+        let second = st;
+
+        // trace!("rtc: instant now: st, ssr: {}, {}", st, ssr);
+
+        Self { second, subsecond }
+    }
+}
+
+#[cfg(feature = "low-power")]
+impl core::ops::Sub for RtcInstant {
+    type Output = embassy_time::Duration;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        use embassy_time::{Duration, TICK_HZ};
+
+        let second = if self.second < rhs.second {
+            self.second + 60
+        } else {
+            self.second
+        };
+
+        // TODO: read prescaler
+
+        let self_ticks = second as u32 * 256 + (255 - self.subsecond as u32);
+        let other_ticks = rhs.second as u32 * 256 + (255 - rhs.subsecond as u32);
+        let rtc_ticks = self_ticks - other_ticks;
+
+        //        trace!(
+        //            "rtc: instant sub: self, other, rtc ticks: {}, {}, {}",
+        //            self_ticks,
+        //            other_ticks,
+        //            rtc_ticks
+        //        );
+
+        Duration::from_ticks(((rtc_ticks * TICK_HZ as u32) / 256u32) as u64)
+    }
+}
+
 /// RTC Abstraction
 pub struct Rtc {
     rtc_config: RtcConfig,
+    #[cfg(feature = "low-power")]
+    stop_time: Mutex<CriticalSectionRawMutex, Cell<Option<RtcInstant>>>,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq)]
@@ -108,8 +180,15 @@ impl Rtc {
     pub fn new(_rtc: impl Peripheral<P = RTC>, rtc_config: RtcConfig) -> Self {
         RTC::enable_peripheral_clk();
 
+        #[cfg(not(feature = "low-power"))]
         let mut rtc_struct = Self { rtc_config };
 
+        #[cfg(feature = "low-power")]
+        let mut rtc_struct = Self {
+            rtc_config,
+            stop_time: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
+        };
+
         Self::enable();
 
         rtc_struct.configure(rtc_config);
diff --git a/embassy-stm32/src/rtc/v2.rs b/embassy-stm32/src/rtc/v2.rs
index bcb127ecb..bf926f986 100644
--- a/embassy-stm32/src/rtc/v2.rs
+++ b/embassy-stm32/src/rtc/v2.rs
@@ -1,67 +1,12 @@
 use stm32_metapac::rtc::vals::{Init, Osel, Pol};
 
+#[cfg(feature = "low-power")]
+use super::RtcInstant;
 use super::{sealed, RtcClockSource, RtcConfig};
 use crate::pac::rtc::Rtc;
 use crate::peripherals::RTC;
 use crate::rtc::sealed::Instance;
 
-#[cfg(all(feature = "time", any(stm32wb, stm32f4)))]
-pub struct RtcInstant {
-    ssr: u16,
-    st: u8,
-}
-
-#[cfg(all(feature = "time", any(stm32wb, stm32f4)))]
-impl RtcInstant {
-    pub fn now() -> Self {
-        // TODO: read value twice
-        use crate::rtc::bcd2_to_byte;
-
-        let tr = RTC::regs().tr().read();
-        let tr2 = RTC::regs().tr().read();
-        let ssr = RTC::regs().ssr().read().ss();
-        let ssr2 = RTC::regs().ssr().read().ss();
-
-        let st = bcd2_to_byte((tr.st(), tr.su()));
-        let st2 = bcd2_to_byte((tr2.st(), tr2.su()));
-
-        assert!(st == st2);
-        assert!(ssr == ssr2);
-
-        let _ = RTC::regs().dr().read();
-
-        trace!("ssr: {}", ssr);
-        trace!("st: {}", st);
-
-        Self { ssr, st }
-    }
-}
-
-#[cfg(all(feature = "time", any(stm32wb, stm32f4)))]
-impl core::ops::Sub for RtcInstant {
-    type Output = embassy_time::Duration;
-
-    fn sub(self, rhs: Self) -> Self::Output {
-        use embassy_time::{Duration, TICK_HZ};
-
-        let st = if self.st < rhs.st { self.st + 60 } else { self.st };
-
-        // TODO: read prescaler
-
-        let self_ticks = st as u32 * 256 + (255 - self.ssr as u32);
-        let other_ticks = rhs.st as u32 * 256 + (255 - rhs.ssr as u32);
-        let rtc_ticks = self_ticks - other_ticks;
-
-        trace!("self, other, rtc ticks: {}, {}, {}", self_ticks, other_ticks, rtc_ticks);
-
-        Duration::from_ticks(
-            ((((st as u32 * 256 + (255u32 - self.ssr as u32)) - (rhs.st as u32 * 256 + (255u32 - rhs.ssr as u32)))
-                * TICK_HZ as u32) as u32
-                / 256u32) as u64,
-        )
-    }
-}
-
 #[allow(dead_code)]
 #[derive(Clone, Copy, Debug)]
 pub(crate) enum WakeupPrescaler {
@@ -144,15 +89,10 @@ impl super::Rtc {
         }
     }
 
-    #[allow(dead_code)]
-    #[cfg(all(feature = "time", any(stm32wb, stm32f4)))]
-    /// start the wakeup alarm and return the actual duration of the alarm
-    /// the actual duration will be the closest value possible that is less
-    /// than the requested duration.
-    ///
-    /// note: this api is exposed for testing purposes until low power is implemented.
-    /// it is not intended to be public
-    pub(crate) fn start_wakeup_alarm(&self, requested_duration: embassy_time::Duration) -> RtcInstant {
+    #[cfg(feature = "low-power")]
+    /// start the wakeup alarm and wtih a duration that is as close to but less than
+    /// the requested duration, and record the instant the wakeup alarm was started
+    pub(crate) fn start_wakeup_alarm(&self, requested_duration: embassy_time::Duration) {
         use embassy_time::{Duration, TICK_HZ};
 
         use crate::rcc::get_freqs;
@@ -174,37 +114,45 @@ impl super::Rtc {
             rtc_ticks as u64 * TICK_HZ * (<WakeupPrescaler as Into<u32>>::into(prescaler) as u64) / rtc_hz,
         );
 
-        trace!("set wakeup timer for {} ms", duration.as_millis());
-
         self.write(false, |regs| {
-            regs.cr().modify(|w| w.set_wutie(true));
-
             regs.cr().modify(|w| w.set_wute(false));
             regs.isr().modify(|w| w.set_wutf(false));
             while !regs.isr().read().wutwf() {}
 
             regs.cr().modify(|w| w.set_wucksel(prescaler.into()));
             regs.cr().modify(|w| w.set_wute(true));
+            regs.cr().modify(|w| w.set_wutie(true));
         });
 
-        RtcInstant::now()
+        trace!("rtc: start wakeup alarm for {} ms", duration.as_millis());
+
+        critical_section::with(|cs| assert!(self.stop_time.borrow(cs).replace(Some(RtcInstant::now())).is_none()))
     }
 
-    #[allow(dead_code)]
-    #[cfg(all(feature = "time", any(stm32wb, stm32f4)))]
-    /// stop the wakeup alarm and return the time remaining
-    ///
-    /// note: this api is exposed for testing purposes until low power is implemented.
-    /// it is not intended to be public
-    pub(crate) fn stop_wakeup_alarm(&self) -> RtcInstant {
-        trace!("disable wakeup timer...");
+    #[cfg(feature = "low-power")]
+    /// stop the wakeup alarm and return the time elapsed since `start_wakeup_alarm`
+    /// was called, otherwise none
+    pub(crate) fn stop_wakeup_alarm(&self) -> Option<embassy_time::Duration> {
+        use crate::interrupt::typelevel::Interrupt;
+
+        trace!("rtc: stop wakeup alarm...");
 
         self.write(false, |regs| {
+            regs.cr().modify(|w| w.set_wutie(false));
             regs.cr().modify(|w| w.set_wute(false));
             regs.isr().modify(|w| w.set_wutf(false));
+
+            crate::pac::EXTI.pr(0).modify(|w| w.set_line(22, true));
+            crate::interrupt::typelevel::RTC_WKUP::unpend();
         });
 
-        RtcInstant::now()
+        critical_section::with(|cs| {
+            if let Some(stop_time) = self.stop_time.borrow(cs).take() {
+                Some(RtcInstant::now() - stop_time)
+            } else {
+                None
+            }
+        })
     }
 
     #[allow(dead_code)]
diff --git a/embassy-stm32/src/time_driver.rs b/embassy-stm32/src/time_driver.rs
index 2622442f4..99d423d08 100644
--- a/embassy-stm32/src/time_driver.rs
+++ b/embassy-stm32/src/time_driver.rs
@@ -14,6 +14,8 @@ use stm32_metapac::timer::regs;
 use crate::interrupt::typelevel::Interrupt;
 use crate::pac::timer::vals;
 use crate::rcc::sealed::RccPeripheral;
+#[cfg(feature = "low-power")]
+use crate::rtc::Rtc;
 use crate::timer::sealed::{Basic16bitInstance as BasicInstance, GeneralPurpose16bitInstance as Instance};
 use crate::{interrupt, peripherals};
 
@@ -130,12 +132,14 @@ impl AlarmState {
     }
 }
 
-struct RtcDriver {
+pub(crate) struct RtcDriver {
     /// Number of 2^15 periods elapsed since boot.
     period: AtomicU32,
     alarm_count: AtomicU8,
     /// Timestamp at which to fire alarm. u64::MAX if no alarm is scheduled.
     alarms: Mutex<CriticalSectionRawMutex, [AlarmState; ALARM_COUNT]>,
+    #[cfg(feature = "low-power")]
+    rtc: Mutex<CriticalSectionRawMutex, Cell<Option<&'static Rtc>>>,
 }
 
 const ALARM_STATE_NEW: AlarmState = AlarmState::new();
@@ -144,6 +148,8 @@ embassy_time::time_driver_impl!(static DRIVER: RtcDriver = RtcDriver {
     period: AtomicU32::new(0),
     alarm_count: AtomicU8::new(0),
     alarms: Mutex::const_new(CriticalSectionRawMutex::new(), [ALARM_STATE_NEW; ALARM_COUNT]),
+    #[cfg(feature = "low-power")]
+    rtc: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
 });
 
 impl RtcDriver {
@@ -259,6 +265,117 @@ impl RtcDriver {
         let f: fn(*mut ()) = unsafe { mem::transmute(alarm.callback.get()) };
         f(alarm.ctx.get());
     }
+
+    #[cfg(feature = "low-power")]
+    /// Set the rtc but panic if it's already been set
+    pub(crate) fn set_rtc(&self, rtc: &'static Rtc) {
+        critical_section::with(|cs| assert!(self.rtc.borrow(cs).replace(Some(rtc)).is_none()));
+    }
+
+    #[cfg(feature = "low-power")]
+    /// Compute the approximate amount of time until the next alarm
+    fn time_until_next_alarm(&self) -> embassy_time::Duration {
+        critical_section::with(|cs| {
+            let now = self.now() + 32;
+
+            embassy_time::Duration::from_ticks(
+                self.alarms
+                    .borrow(cs)
+                    .iter()
+                    .map(|alarm: &AlarmState| alarm.timestamp.get().saturating_sub(now))
+                    .min()
+                    .unwrap_or(u64::MAX),
+            )
+        })
+    }
+
+    #[cfg(feature = "low-power")]
+    /// Add the given offset to the current time
+    fn add_time(&self, offset: embassy_time::Duration) {
+        let offset = offset.as_ticks();
+        let cnt = T::regs_gp16().cnt().read().cnt() as u32;
+        let period = self.period.load(Ordering::SeqCst);
+
+        // Correct the race, if it exists
+        let period = if period & 1 == 1 && cnt < u16::MAX as u32 / 2 {
+            period + 1
+        } else {
+            period
+        };
+
+        // Normalize to the full overflow
+        let period = (period / 2) * 2;
+
+        // Add the offset
+        let period = period + 2 * (offset / u16::MAX as u64) as u32;
+        let cnt = cnt + (offset % u16::MAX as u64) as u32;
+
+        let (cnt, period) = if cnt > u16::MAX as u32 {
+            (cnt - u16::MAX as u32, period + 2)
+        } else {
+            (cnt, period)
+        };
+
+        let period = if cnt > u16::MAX as u32 / 2 { period + 1 } else { period };
+
+        self.period.store(period, Ordering::SeqCst);
+        T::regs_gp16().cnt().write(|w| w.set_cnt(cnt as u16));
+
+        // Now, recompute all alarms
+        critical_section::with(|cs| {
+            for i in 0..ALARM_COUNT {
+                let alarm_handle = unsafe { AlarmHandle::new(i as u8) };
+                let alarm = self.get_alarm(cs, alarm_handle);
+
+                self.set_alarm(alarm_handle, alarm.timestamp.get());
+            }
+        })
+    }
+
+    #[cfg(feature = "low-power")]
+    /// Stop the wakeup alarm, if enabled, and add the appropriate offset
+    fn stop_wakeup_alarm(&self) {
+        critical_section::with(|cs| {
+            if let Some(offset) = self.rtc.borrow(cs).get().unwrap().stop_wakeup_alarm() {
+                self.add_time(offset);
+            }
+        });
+    }
+
+    #[cfg(feature = "low-power")]
+    /// Pause the timer if ready; return err if not
+    pub(crate) fn pause_time(&self) -> Result<(), ()> {
+        /*
+            If the wakeup timer is currently running, then we need to stop it and
+            add the elapsed time to the current time
+        */
+        self.stop_wakeup_alarm();
+
+        let time_until_next_alarm = self.time_until_next_alarm();
+        if time_until_next_alarm < embassy_time::Duration::from_millis(250) {
+            Err(())
+        } else {
+            critical_section::with(|cs| {
+                self.rtc
+                    .borrow(cs)
+                    .get()
+                    .unwrap()
+                    .start_wakeup_alarm(time_until_next_alarm);
+            });
+
+            T::regs_gp16().cr1().modify(|w| w.set_cen(false));
+
+            Ok(())
+        }
+    }
+
+    #[cfg(feature = "low-power")]
+    /// Resume the timer with the given offset
+    pub(crate) fn resume_time(&self) {
+        self.stop_wakeup_alarm();
+
+        T::regs_gp16().cr1().modify(|w| w.set_cen(true));
+    }
 }
 
 impl Driver for RtcDriver {
@@ -329,6 +446,11 @@ impl Driver for RtcDriver {
     }
 }
 
+#[cfg(feature = "low-power")]
+pub(crate) fn get_driver() -> &'static RtcDriver {
+    &DRIVER
+}
+
 pub(crate) fn init() {
     DRIVER.init()
 }
diff --git a/tests/stm32/Cargo.toml b/tests/stm32/Cargo.toml
index 754356cb5..1f8c7373c 100644
--- a/tests/stm32/Cargo.toml
+++ b/tests/stm32/Cargo.toml
@@ -7,7 +7,7 @@ autobins = false
 
 [features]
 stm32f103c8 = ["embassy-stm32/stm32f103c8", "not-gpdma"]     # Blue Pill
-stm32f429zi = ["embassy-stm32/stm32f429zi", "chrono", "can", "not-gpdma", "dac-adc-pin"]     # Nucleo "sdmmc"
+stm32f429zi = ["embassy-stm32/stm32f429zi", "chrono", "stop", "can", "not-gpdma", "dac-adc-pin"]     # Nucleo "sdmmc"
 stm32g071rb = ["embassy-stm32/stm32g071rb", "not-gpdma", "dac-adc-pin"]     # Nucleo
 stm32c031c6 = ["embassy-stm32/stm32c031c6", "not-gpdma"]     # Nucleo
 stm32g491re = ["embassy-stm32/stm32g491re", "not-gpdma"]     # Nucleo
@@ -17,6 +17,7 @@ stm32h563zi = ["embassy-stm32/stm32h563zi"]     # Nucleo
 stm32u585ai = ["embassy-stm32/stm32u585ai"]     # IoT board
 
 sdmmc = []
+stop = ["embassy-stm32/low-power"]
 chrono = ["embassy-stm32/chrono", "dep:chrono"]
 can = []
 ble = ["dep:embassy-stm32-wpan", "embassy-stm32-wpan/ble"]
@@ -47,6 +48,7 @@ micromath = "2.0.0"
 panic-probe = { version = "0.3.0", features = ["print-defmt"] }
 rand_core = { version = "0.6", default-features = false }
 rand_chacha = { version = "0.3", default-features = false }
+static_cell = {version = "1.1", features = ["nightly"] }
 
 chrono = { version = "^0.4", default-features = false, optional = true}
 
@@ -88,6 +90,11 @@ path = "src/bin/spi_dma.rs"
 required-features = []
 
 [[bin]]
+name = "stop"
+path = "src/bin/stop.rs"
+required-features = [ "stop", "chrono",]
+
+[[bin]]
 name = "timer"
 path = "src/bin/timer.rs"
 required-features = []
diff --git a/tests/stm32/src/bin/stop.rs b/tests/stm32/src/bin/stop.rs
new file mode 100644
index 000000000..4a49bde9d
--- /dev/null
+++ b/tests/stm32/src/bin/stop.rs
@@ -0,0 +1,53 @@
+// required-features: stop,chrono
+
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+#[path = "../common.rs"]
+mod common;
+
+use chrono::NaiveDate;
+use common::*;
+use cortex_m_rt::entry;
+use embassy_executor::Spawner;
+use embassy_stm32::low_power::{stop_with_rtc, Executor};
+use embassy_stm32::rtc::{Rtc, RtcClockSource, RtcConfig};
+use embassy_time::{Duration, Timer};
+use static_cell::make_static;
+
+#[entry]
+fn main() -> ! {
+    let executor = Executor::take();
+    executor.run(|spawner| {
+        unwrap!(spawner.spawn(async_main(spawner)));
+    });
+}
+
+#[embassy_executor::task]
+async fn async_main(_spawner: Spawner) {
+    let mut config = config();
+
+    config.rcc.rtc = Some(RtcClockSource::LSI);
+
+    let p = embassy_stm32::init(config);
+    info!("Hello World!");
+
+    let now = NaiveDate::from_ymd_opt(2020, 5, 15)
+        .unwrap()
+        .and_hms_opt(10, 30, 15)
+        .unwrap();
+
+    let mut rtc = Rtc::new(p.RTC, RtcConfig::default());
+
+    rtc.set_datetime(now.into()).expect("datetime not set");
+
+    let rtc = make_static!(rtc);
+
+    stop_with_rtc(rtc);
+
+    info!("Waiting 5 seconds");
+    Timer::after(Duration::from_secs(5)).await;
+
+    info!("Test OK");
+    cortex_m::asm::bkpt();
+}